Field of the Invention
This invention relates generally to light emitting devices, such as organic light emitting diodes, and, more particularly, to a light emitting device having a light scattering surface and to methods of making the light emitting device.
Technical Considerations
An organic light emitting diode (OLED) is an example of a light-emitting device. An OLED has a multilayer structure incorporating an active stack having a thin organic film, e.g., an electroluminescent emissive layer of organic semiconductor material. The active stack is located between two electrodes (an anode and a cathode). When electric current is passed between the anode and the cathode, the emissive layer emits light, typically visible light in response to the application of an electric current. OLEDs are used in numerous applications, such as television screens, computer monitors, mobile phones, PDAs, watches, lighting, and various other electronic devices.
OLEDs provide numerous advantages over conventional inorganic light emitting devices, such as liquid crystals and Incandescent or compact fluorescent lamps (CLFs). For example, an OLED functions without the need for a back light. In low ambient light, such as a dark room, an OLED screen can achieve a higher contrast ratio than conventional liquid crystal displays. OLEDs are also thinner, lighter, and more flexible than liquid crystal displays. OLEDs require less energy to operate and provide cost savings compared to incandescent or compact fluorescent lamps.
However, one disadvantage with OLEDs is that a significant amount of light generated by the active stack is lost due to the optical waveguide effect created by the refractive index differences between the various layers of the OLED. A portion of the light emitted by the emissive layer is reflected back at the various layer boundaries and becomes trapped within the layers. In a conventional OLED lighting device, about 80% of the visible light emitted from the organic emissive layer is trapped inside the OLED due to this optical waveguide effect.
Therefore, it would be advantageous to provide a device and/or method to extract more electromagnetic radiation, e.g., visible light, from an OLED than is possible in conventional OLEDs.
Currently, increased light extraction methods involve a combination of complex coating application methods and surface patterning methods. For example, the outer surface of the OLED substrate can be chemically etched (such as acid etched) or physically etched (such as by tools) to increase the surface roughness of the substrate surface. Increasing the surface roughness increases haze, which generally increases the amount of light extracted from the OLED. However, these current coating and patterning processes increase the manufacturing time and cost required to make the OLED and may lead to environmental concerns in storage and disposal of the etching and/or coating materials. Additionally, these methods do nothing to affect the waveguide effect between the various internal layers of the OLED.
Therefore, it also would be advantageous to provide a light emitting device, such as an OLED, and/or a method of making a light emitting device, having one or more of the following advantages over current devices or methods: reduced waveguide effect; higher light emission; simpler manufacturing process; reduced production cost; fewer production steps; and/or less complex production steps.